Slow mail with MTAHave you actually tested sending several mails or are you only measuring the performance of a single mail at a time? Perhaps sending one takes the same time as sending 1000? Try to test with what you expect to be a 'real world' scenario.

Set up spf and dkim, where email domain and sending domain differDKIM will generate a hash from some of the email headers, when receiving this email, the receiving side will look up the key on the sender's DNS server (foo.com on your case) and use that to decrypt and 'checksum' the header. So if you setup the signing keys on your bar.com server, you can sign email for foo.com and use DKIM. Yahoo is particularly fond of DKIM. I would start with the SPF and implement DKIM if I still had issues after that.

Sending email from a Google Apps addressSo Google is picking up that IPv6 address as the address of your server? Can you paste the whole email headers? It looks like your postfix server may be misconfigured. ~all leaves the choice on the hands of the receiving ISP. If you send from both google and your server, include both IP4 address of your server and google's spf include.